1. Field of the Invention
The present invention relates generally to the art of packaging microcircuits and particularly high power hybrid microcircuit packages. In this technology, several thin-film semiconductor devices are placed in one microcircuit package. The interconnects are accomplished by placing the semiconductor devices on ceramic substrates printed with thick-film or thin-film metallized tracks. Hence, this is a "hybrid" semiconductor containing several devices of both thick and thin-film technologies all contained in one microcircuit package. This invention specifically pertains to the hybrid package itself, particularly useful in packaging several high power semiconductor devices.
2. Description of the Prior Art
In the current art of building metal hybrid microcircuit packages, the most common metals being used are cold rolled steel, stainless steel, molybdenum, aluminum, copper, and Kovar (a very well-known iron-nickel-cobalt alloy). The most popular and technically advantageous metal to build a microcircuit package with is Kovar. The normal Kovar package is a box-shaped enclosure with a single piece side wall that is high-temperature brazed to the package bottom. Alternatively, the bottom and side wall are stamped from a single sheet or Kovar material. Holes are then drilled or punched into the bottom, e.g. in the familiar dual-in-line or plug-in configurations or in the side wall, e.g. in the flat pack or butterfly configuration. The leads, also most commonly made of Kovar, are glass-sealed into the package to complete the assembly. The glass serves both to insulate the leads from the body and to form a hermetic seal.
The typical application of this package is to epoxy or solder the ceramic substrate containing the semiconductor devices to the bottom of the package, wire the thick-film metal tracks to the leads, as required, and hermetically weld a Kovar lid to seal the package. This assembly, known as a hybrid microcircuit, can then be soldered onto a printed wiring board and used similar to any ordinary discrete microcircuit containing only a single semiconductor device. The advantage of a hybrid microcircuit is a decrease in weight and volume over the equivalent number of discrete devices. The main advantage of using Kovar for the package is that its coefficient of thermal expansion is similar to both the ceramic substrates and the glass seals. Consequently, the complete assembly expands and contracts at the same rate. No excessive thermal stresses, therefore, are developed in the assembly during temperature extremes of fabrication and environmental testing. Without thermally matching materials, damage from stresses occurs, such as cracks in substrates, glass seals, weld joints, braze joints resulting in loss of hermeticity. Hermeticity requirements, internal atmosphere requirements, and no loose internal particles are the main reason metal packages are chosen over ceramic or plastic type packages.
Of particular interest and advantage in using metal packages is that they can be sealed in a conventional seam sealing machine which ensures control over the internal atmosphere. A seam sealer consists of three internally connected chambers. The first (entrance) chamber is a vacuum bake oven where water, cleaning chemicals, and other contaminants are evaporated and drawn away from the parts. The center (welding) chamber, which is backfilled with dry nitrogen, contains the welding apparatus. The third (exit) chamber is a double door pass-through purged with dry nitrogen to prevent back streaming of air and water vapor into the welding chamber. The parts are passed directly from the vacuum bake chamber to the welding chamber. The lid and package are hermetically welded together (sealing in the dry nitrogen) by a series of overlapping spot welds. There are no loose internal particles created with this system as can occur when using a solder or epoxy. Also, there is no flux needed as with some solders which would contaminate the internal atmosphere. And finally, the joint is environmentally stronger than either soldering or epoxying on the lid.
The major disadvantage of Kovar, however, is that it has a low thermal conductivity. The use of Kovar microcircuit packages is, therefore, limited to the packaging of low power semiconductor devices. The maximum electrical power a Kovar package can dissipate is approximately one watt per square inch without overheating the semiconductor device and adversely affecting their electrical characteristics. Metals with higher thermal conductivity, like cold rolled steel, molybdenum, aluminum, and copper are, therefore, often used for hybrid microcircuit packaging of high power semiconductor devices. In some applications, copper is the only practical material with a high enough thermal conductivity to dissipate the heat generated by several high power semiconductor devices packed as densely as they typically are in a hybrid.
Use of copper in the prior art has, however, disadvantages which must be considered if it is used as a hybrid package material. First, its coefficient of thermal expansion is considerably different from the ceramic substrates and glass seals. An assembled hybrid microcircuit cannot be designed, therefore, which expands and contracts at the same rate, resulting in an assembly free from thermal stresses. Secondly, copper has an annealing temperature of 375.degree. C. If processed above this temperature, as typically necessary in the prior art with high temperature brazing, copper will change from a relatively elastic to a plastic, or inelastic material. Like all plastic materials, any force which causes stresses in excess of the material's yield strength will cause a permanent physical deformation of the part. Such deformation causes cracked substrates and loss of hermeticity in the assembled device. In addition, annealed copper can only be strengthened by hardening through cold working the copper, which is not practical with a machined part. Beryllium can be added to the copper to make it hardenable by heat treating processes, but even adding a small amount of beryllium reduces its thermal conductivity which significantly reduces the advantages of the material. Finally, hermetically sealing a lid to a copper bodied package must be limited to a low temperature soldering or brazing process. Such a process requires care in selecting a material that either melts at a temperature low enough that does not damage other internal assemblies, or is localized enough to prevent exposure of the internal assemblies to excessive heating. Localized heating can be accomplished with specialized equipment, which may often be impractical or not available. Equipment for perimeter, or seam sealing equipment, typically is available, and can be used for soldering, but localized heating requires using a lid of low thermal conductivity, such as Kovar, to effect the contact resistance heating. This, of course, results in another thermal mismatch between the lid and copper body, which will stress the weaker solder alloy causing evenual hermeticity failures.
Three approaches have been followed in the past to overcome the problems of using copper as a microcircuit hybrid package material. The first is to machine the base including bottom and side walls out of copper. Lead assemblies are then low-temperature brazed (under 375.degree. C.) into the package. A lead assembly consists of a lead, a metal shell, or eyelet, and a glass seal between the lead and shell. Lead assemblies are used when leads cannot be directly glass sealed into the side wall or bottom of a package due to thermal mismatch of materials or deleterious effects of the high temperature, e.g., annealing of copper. This copper package configuration has a major disadvantage of not having a suitable top surface for seam welding a lid in place to ensure a strong hermetic device without risk of internal contamination.
To overcome the disadvantage of the first approach, a second prior art approach is to low-temperature braze a side wall or seal ring of low thermal conductivity metal onto a copper bottom. A seal ring is a thin window frame shaped piece of metal which is attached to the top of a copper side wall to provide a low thermal conductivity welding surface. This configuration can, therefore, be sealed in a seam sealer and does have a high thermal conductivity package bottom to remove heat generated by internal components. The disadvantage, however, of this configuration is the low strength of the braze joint, which is necessarily a low temperature braze (less than 375.degree. C.) to ensure the copper is not annealed and weakened. The difficulty with this approach is similar to that of the first approach in that the braze joint is between two very dissimilar materials whose thermal coefficient of expansion differences are enough to compromise the strength of the joint during thermal exposures.
The third prior approach is to high-temperature braze on a seal ring or side wall. This configuration can be sealed in a seam sealer and the braze joint is strong enough to withstand environmental testing. The disadvantage, however, of this configuration is that high temperature brazing is done well above the 375.degree. C. annealing temperature of copper, which compromises its strength causing it to yield to the thermal mismatch of other materials used in the hybrid device assembly; i.e., ceramic substrates.